The eye includes several tissues that allow patients to see. The cornea of the eye is an anterior tissue of the eye that is clear in healthy eyes and refracts light so as to form an image on the retina. The retina is a posterior tissue of the eye that senses light from the image formed thereon and transmits signals from the image to the brain. The cornea includes an outer layer of tissue, the epithelium, which protects the underlying tissues of the cornea, such as Bowman's membrane, the stroma, and nerve fibers that extend into the stroma and Bowman's membrane. The healthy eye includes a tear film disposed over the epithelium. The tear film can smooth small irregularities of the epithelium so as to provide an optically smooth surface and maintain eye health. The tear film is shaped substantially by the shape of the underlying epithelium, stroma, and Bowman's membrane, if present. The tear film comprises a liquid that is mostly water and does include additional components, such as mucoids and lipids. The many nerve fibers of the cornea provide sensation to promote blinking that can cover the cornea with the tear film. The never fibers also sense pain so that one will normally avoid trauma to the cornea and also avoid direct contact of an object to the cornea so as to protect this important tissue.
In the healthy cornea, the proper amount of hydration of the cornea, sometimes referred to as dehydration of the cornea, is maintained such that the cornea remains clear. The cornea includes a posterior endothelial layer that pumps water from the cornea into the adjacent anterior chamber. The epithelium inhibits flow of water from the tear liquid into the cornea, such that the corneal stroma can be maintained with the proper amount of hydration with endothelial pumping. The endothelial pumping of water from the cornea to maintain the proper hydration and thickness of the eye is referred to as deturgescence. When the corneal epithelium heals, the layer of cells forming over the defect can be at least somewhat irregular in at least some instances, such that the vision of the patient can be less than ideal.
Following corneal surgery, such as refractive keratectomy, the post-ablation cornea may have a complex shape, and many of the prior commercially available lenses may not fit the ablated cornea as well as would be ideal, and in at least some instances fitting of lenses can be time consuming and awkward. Commercially available contact lenses having a rigid gas permeable (RGP) central portion and a soft peripheral skirt can be difficult and/or time consuming to fit to the ablated cornea and may not fit very well in at least some instances. The ablated cornea may comprise an abrupt change in curvature near the edge of the ablation, and in at least some instances it can be difficult to fit such lenses near the edge of the ablation. Also, at least some of the commercially available contact lenses may not be suitable for extended wear and may be removed each day, which can be somewhat awkward for a patient and can result in lack of compliance and lenses remaining in the eye longer than would be ideal in at least some instances.
Hybrid or bimodular contact lenses, lenses having a comparatively rigid central portion and a soft skirt are also used to correct refractive error of the eye such as astigmatism. Current products such as RGP and soft toric lenses for correcting refractive error include a cylindrical component in addition to any spherical corrective component that must be determined for each patient and oriented with respect to the optical region of the cornea to maintain optimal vision correction. Features are incorporated into the lens to maintain centration and radial orientation of the lens of the eye during wear. Because of the need to fit and orient the cylindrical corrective component, a large number of lenses must be maintained in inventory and individually fit and selected for each patient.
In light of the above, it is desirable to provide improved contact lenses for vision correction and coverings for treatments related to epithelial defects of the cornea, such as epithelial defects following PRK. Ideally, these contact lenses and coverings would provide treatments that improve tear flow and avoid at least some of the deficiencies of known techniques while providing improved patient comfort and/or vision. It is also desirable to provide improved contact lenses for correcting refractive error that only require a spherical fit and provide comfort and vision correction as good as or better than current toric lens products.